Hey,
I built my 18watter from Tube-town in Germany, they are really great.
Here you can find a lot of transformers, different parts, etc.. Also there's a 18watt kit on their site, which contains everything, except the chassis! (Transformers, tubes, parts) Ofc the chassis can be bought here as well, you just have to choose that you need one for a combo or a head.
I'm not sure if it good for you shipping-wise, but worth a look, because they are not that expensive, and every part I got from them is great quality (look at the First t00b amp topic, I posted some pictures there about my builds)
https://www.tube-town.net/ttstore/Transformer/Power-Transformer/Marshall:::15_530_541.htmlhttps://www.tube-town.net/ttstore/Kits/Verstaerker/TT-Kit-Marshall-Style-18-Watt::7360.html